In a car, it's a small light vehicle and customers buy them based on HP numbers / 0-60 times / handling etc.
In a van, it's got to slog away carrying a heavy load, withstand operator abuse, and be economical and reliable for the warranty period regardless of that abuse / poor treatment.
De-tuning does not necessarily mean what you think - HP is based on torque X RPM, but for a heavy vehicle you need low down torque and a good spread (flexibility), not a sports-car "peaky" shove-in-the-back-and-then-shift kinda behaviour. A truck with a racing-car cam in it would be awful to drive.
You're comparing a work horse with a race horse. One can't run fast, but the other can't pull a plough.
They may also limit emissions for similar reasons, if memory serves Skoda did a 1.9TDi that was identical in two vehicles - one was the "sports" which went like stink but emitted lots of CO2 (so higher tax class in Europe) and used more fuel, and one was the "eco" tune which sipped fuel & got you zero-tax and low insurance. I know which one fleet buyers would be going for. Same engine in a VW Transporter I'd wager they drop the HP numbers but up the torque.
Also they electronically limit the torque based on gear as they make gearboxes smaller & lighter (and make 5th/6th out of plastic for cheapness/noise reduction) so you can't have full torque trying to race your fully-laden van off the traffic lights (saves many clutch & gearbox rebuilds under warranty).
My last van (GM/Renault/Nissan Vivaro) shared the GM 1.9TDi with the Vectra et al but hit a brick-wall rev limit at 4krpm, pretty sure none of their cars did that.
Land Rover used to de-tune Defender engines for all these and more: Bad fuel in remote locations, static running (powering PTO units etc.) for long periods in hot places, and to avoid over exuberant driving by employees / squaddies. Often they had 30% less peak-HP than the same engine in a Range Rover.
Some industrial engines (based on car or truck engines) are tuned specifically to work at ONE fixed RPM - 1500 or 3000 for generators, with carefully tuned cam profiles etc. to suit. They can make much better power at that ONE RPM than the other variants but, again, would utterly suck in a vehicle.
I saw a guy who built a 4x4 (SJ410) with a ridiculous race-tuned car engine he found, it utterly sucked - it was all-or-nothing power delivery (zero below 5000rpm, then a million HP), very hard to drive, easy to stall, poor control, it stressed the drivetrain with all the jerking / clutch slipping, and overheated when working hard at low speeds. People in identical vehicles with bone stock engines were able to drive circles round him. Sure he had lots of HP at 5000RPM, very useful for bragging down the pub.
Edit to add: Top Gear demonstrated one example of tradeoffs for larger HP numbers when they reviewed one of the Mitsubishi Evo's. The top of the line most powerful one (400HP?) was awful to drive as all the power was at one point, the lower spec one (~300HP?) actually drove better as it had a broader spread of torque, meaning you didn't have to change gear as much. They raced the 400HP one against their camera car people-carrier (Fiat Multipla?) off the line from tickover and the minivan out-dragged it for a long time until the turbos spun up & everything came into the zone. You could probably find it on YouTube if you're interested.